Because of the dramatic drop in cost of computer semiconductor memory, it is becoming economical and efficient to store image data in computer archival storage and use semiconductor memory as a temporary storage site. For example, in a large hospital hundreds of X-ray film images are made every day. In the past these X-ray films were often stored indefinitely. It has been recognized that computer mass storage of such images is preferable if the image can be reproduced with speed and accuracy. For example, such storage might be on optical or magnetic disks. It is also desirable to digitize images on hard film copies produced by CAT scanning and magnetic resonance imaging applications where the original digital image data has been lost or discarded.
In U.S. Pat. No. 4,236,183, Howe discloses an apparatus for scanning film which includes a solid-state line sensing array for scanning a given line of projected image of the film in a direction generally perpendicular to the length of the film, and a rotating multifacet mirror for displacing the image relative to the line sensing array in a direction generally parallel to the length of the film to effect a raster scan of the film. A light source behind the film and a scan lens project the image portion defined by a film gate onto the sensing array. To reduce distortion the optical axis of the scan lens, the axis of rotation of the multifacet mirror and the center of the film gate lie in a common plane with the multifacet mirror located in the optical path between the scan lens and the film.
There are present efforts devoted to devising image digitizers and reproducers which are both faster and simpler without sacrificing the reproduced image quality of prior devices. For example, a compact scanner/plotter is described in U.S. Pat. No. 4,024,518 to Boston. A photosensitive film is mounted on a rotatable drum. A scanning head moves on a precision leadscrew so as to scan the film in a helical fashion as the drum rotates. The medium receives an exposure that is a function of the gray levels of the image being recorded. Unfortunately, presently available leadscrews are not accurate enough to avoid misregistration between adjacent lines recorded on the medium, which typically are less than 100 microns wide. Further, most drum-type film scanners are slow, typically taking about five minutes to record or digitize an image on a 14 inch by 17 inch sheet of film. It would be desirable to scan an image on such a film sheet in about twenty seconds. However, such a recording speed would require a drum speed of about 15,000 rpm, which is not practical.
An object of the present invention is to produce an apparatus for digitizing an image from a film medium which is both compact, accurate and fast without sacrificing image quality.